1. Field of the Invention
This invention relates to an electric field suppressing cable and a method of using same. In one aspect, the invention relates to an electric field suppressing cable used with devices to analyze geologic formations adjacent a well before completion and a method of using same.
2. Description of Related Art
Generally, geologic formations within the earth that contain oil and/or petroleum gas have properties that may be linked with the ability of the formations to contain such products. For example, formations that contain oil or petroleum gas have higher electrical resistivities than those that contain water. Formations generally comprising sandstone or limestone may contain oil or petroleum gas. Formations generally comprising shale, which may also encapsulate oil-bearing formations, may have porosities much greater than that of sandstone or limestone, but, because the grain size of shale is very small, it may be very difficult to remove the oil or gas trapped therein.
Accordingly, it may be desirable to measure various characteristics of the geologic formations adjacent to a well before completion to help in determining the location of an oil- and/or petroleum gas-bearing formation as well as the amount of oil and/or petroleum gas trapped within the formation. Logging tools, which are generally long, pipe-shaped devices, may be lowered into the well to measure such characteristics at different depths along the well.
These logging tools may include gamma-ray emitters/receivers, caliper devices, resistivity-measuring devices, neutron emitters/receivers, and the like, which are used to sense characteristics of the formations adjacent the well. A wireline cable connects the logging tool with one or more electrical power sources and data analysis equipment at the earth""s surface, as well as providing structural support to the logging tools as they are lowered and raised through the well. Generally, the wireline cable is spooled out of a truck, over a pulley, and down into the well.
As may be appreciated, the diameter of the wireline cable is generally constrained by the handling properties of the cable. For example, a wireline cable having a large diameter may be very difficult to spool and unspool. As a result, many wireline cables have diameters that are generally less than about 13 mm, and thus have a fixed cross-sectional area through which to run conductors for transmitting power to the logging tools and for transmitting data signals from the logging tools. Further, such cables may have lengths of up to about 10,000 m so that the logging tools may be lowered over the entire depth of the well.
Long cable lengths, in combination with small conductors (e.g., 14 AWG to 22 AWG) within the cables, may lead to significant electrical losses, resulting in a reduction in the power received by the logging tools and distortion or attenuation of the data signals transmitted from the logging tools. Further, as logging tools have evolved, the power required to operate the tools has increased. However, the power-transmitting capacity of such cables is limited by the conductor size and the voltage rating of the conductor. Thus, a need exists for cables that are capable of conducting larger amounts of power while reducing undesirable electrical effects induced in both the electrical power and data signals transmitted over the conductors of the cable.
Further, conventional wireline cables may use layers of metallic armor wires that encase the exterior of the wireline cable as a return for electrical power transmitted to the logging tools so that conductors internal to the cable may be used for power and data transmission. Such configurations may present a hazard to personnel and equipment that inadvertently come into contact with the armor wires during operation of the logging tools. Thus, a need exists for a wireline cable that avoids using the metallic armor as an electrical return.
Such problems are also faced in other applications in which the size of electrical cables is constrained and increased electrical power is desired, such as in marine and seismic applications. The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems detailed above.
In one aspect of the present invention, a cable is provided. The cable includes an electrical conductor, a first insulating jacket disposed adjacent the electrical conductor and having a first relative permittivity, and a second insulating jacket disposed adjacent the first insulating jacket and having a second relative permittivity that is less than the first relative permittivity.
In another aspect of the present invention, a method is provided including providing an electrical conductor coupled to a device and having a multi-layered insulating jacket capable of suppressing an electrical field induced by a voltage applied to the electrical conductor and conducting an electrical current through the conductor to or from the device.
In yet another aspect of the present invention, a method is provided for manufacturing a cable. The method includes providing an electrical conductor, extruding a first insulating jacket having a first relative permittivity over the electrical conductor, and extruding a second insulating jacket having a second relative permittivity over the electrical conductor, wherein the second relative permittivity is less than the first relative permittivity.